Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A "light" product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is then delivered from the second end of the bed. A "heavy" product enriched in the more strongly adsorbed component is exhausted from the first end of the bed. The light product is usually the desired product to be purified by PSA, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product is a desired product in the example of nitrogen separation over nitrogen-selective zeolite adsorbents. Typically, the feed is admitted to the first end of a bed and the second product delivered from the second end of the bed when the pressure in that bed is elevated to a higher working pressure, while the second product is exhausted from the first end of the bed at a lower working pressure which is the low pressure of the cycle.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. This conventional pressure swing adsorption process makes inefficient use of applied energy, because of irreversible expansion over the valves over large pressure differences while switching the adsorbent beds between higher and lower pressures.
Keefer (U.S. Pat. No. 5,256,172) discloses the use of expansion turbines to recover power by the principle of thermally coupled pressure swing adsorption, in which expansion energy of the PSA cycle is recovered and heat may be applied directly through an integrated regenerative thermodynamic cycle (regenerative Brayton cycle, or a modified Ericsson cycle) as a supplemental energy source to perform pressure swing adsorption gas separations.
Schartz (PCT publication WO 94/04249) and Firey (U.S. Pat. No. 4,530,705) disclose the use of expanders for partial recovery of energy from countercurrent blowdown gas.
Hay (U.S. Pat. No. 5,246,676) and Engler (U.S. Pat. No. 5,393,326) provide examples of vacuum swing adsorption systems, in which a plurality of vacuum pumps are used to pump down each adsorbent bed being regenerated sequentially in turn, with the pumps operating at successively lower pressures, so that each vacuum pump pumps one bed at a time as the pressure in that bed decreases over a pressure interval, during a pumping time interval for each bed typically equal to the cycle period divided by the number of beds. In these and other prior art devices, the vacuum pumps are subject to varying pressure load over each pumping step on a bed, so that the machines are loaded severely by the cyclically changing duty as well as operating on average less efficiently than under true steady state conditions.
With relatively low PSA cycle frequencies attainable with conventional granular adsorbent beds, the adsorber pressure vessels are bulky and costly. One approach addressing this problem is use of rigid high surface area adsorbent supports which can overcome the limitations of granular adsorbent and enable much higher cycle frequencies. High surface area laminated sheet adsorbent supports, comprised of stacked or spirally wound adsorbent-impregnated sheet material, are disclosed in Keefer's U.S. Pat. Nos. 4,801,308; 4,968,329; and 5,082,473.
For large industrial PSA systems, mechanical immobilization of the adsorbent beds has not been practicable. Careful flow control is required to ensure that pressure gradients in the adsorbent bed are kept low, well below the onset of fluidization.
A further limitation to the use of finely granular adsorbent beds for PSA and other gas separation processes arises as increasingly smaller particle diameters are considered in order to reduce the macropore diffusion mass transfer resistance as required for higher frequency operation. It is well known (as outlined by D. M. Ruthven in "Principles of Adsorption and Adsorption Processes", Wiley, 1984, pages 210-211) that, owing to a tendency of very small particles to cluster and pack unevenly, "the advantage of reduced pore diffusional resistance which is gained by reduction of particle size can easily be offset by increased axial dispersion" for beds packed of small particles.
As operation of PSA processes at high frequencies requires small particle sizes to reduce the diffusional time constant, while the increased axial dispersion prevents a reduction of bed length commensurate with smaller particle diameter, performance tends to degrade due to high pressure drop and bed attrition problems. Hence, cycle frequencies much above 20 cycles per minute have been impracticable in sustained industrial applications, except by use of laminated sheet adsorbent support as mentioned above.
Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784), Petit et al (U.S. Pat. No. 5,441,559) and Schartz (PCT publication WO 94/04249) disclose PSA devices using rotary adsorbent bed configurations. Ports for multiple angularly separated adsorbent beds mounted on a rotor assembly sweep past fixed ports for feed admission, product delivery and pressure equalization; with the relative rotation of the ports providing the function of a rotary distributor valve. Related devices are disclosed by Kagimoto et al (U.S. Pat. No. 5,248,325) and LaCava et al (U.S. Pat. No. 5,487,775). All of these prior art devices use multiple adsorbent beds operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbent beds.
However, a rotary adsorbent bed assembly based on these prior art devices may be impracticable for large PSA units, owing to the weight of the rotating assembly. Also, when separating gas components which are highly inflammable or toxic, the rotary adsorbent bed assembly would need to be completely enclosed in a containment shroud to capture any leakage from large diameter rotary seals. Hence, PSA devices with stationary adsorbent beds have remained referred for larger scale systems, and for applications processing hazardous gases such as hydrogen.
Conventional PSA systems have considerable dead volume associated with the volume of pressure vessel heads, flow distributors and conduit pipework between the ends of the adsorbent column and the directional valves controlling the PSA cycle steps. Prior art rotary PSA devices also have considerable dead volume for flow distribution and collection, since the valve faces are again remote from the adsorbent bed ends. This problem would become proportionately worse if operation at higher cycle frequency were contemplated, since flow distribution would become more critical. These limitations would be especially severe for rotary PSA devices with radial flow through the adsorbent beds, and using either barrel or face valves whose valve surfaces are far separated from the adsorbers as illustrated by the devices of Boudet et al (U.S. Pat. No. 5,133,784), Petit et al (U.S. Pat. No. 5,441,559) and Schartz (PCT publication WO 94/04249).